Not only is Brooke Astor’s son about to go to prison for stealing millions from his Alzheimer’s-afflicted mom, he and his wife have stiffed their lawyers of nearly $5 million in legal fees, The Post has learned.

Anthony Marshall’s defense attorneys have separately filed court papers charging that the couple is $4.7 million in debt to them — and that figure will more than double by the time Marshall is through fighting his case on appeal, which could take years, said a source connected to the case.

Marshall’s wife, Charlene, has personally signed on as a guarantor of the debt, court papers say, meaning that if her already-ailing husband dies before it gets paid, it becomes her burden to bear.

“She didn’t have to agree to do that, but she wanted to help her husband,” the source said.

Marshall’s legal-welching began back in December, more than three months before the trial even began, the court papers, filed in Manhattan Supreme Court, reveal.

That’s when Marshall — faced with a new, $1.3 million bill from Warner Partners, the firm of defense attorney Ken Warner — was only able to cough up a paltry $300,000.

Soon, Marshall was filing “confessions of judgments” — the deadbeat defendant’s version of an IOU — with the court on a regular basis.

Fast on the heels of his $1 million confession of judgment to Warner came another to his other retained law firm, Hafetz and Necheles, admitting he owed $1.3 million.

Charlene — whose current net worth is estimated at $3 million — is on the hook for potentially over $11 million.